This research proposal outlines the synthesis of novel cryptand ligands which incorporate thiolate binding sites and which are designed to bind two metal ions in distinct but closely positioned cavities. The immediate purpose is the preparation of dicopper complexes with these cryptate ligands to simulate the dicopper redox center (CuA site) known to exist in the enzymes cytochrome c oxidase and nitrous oxide reductase. Defining properties of the CuA site are reversible electrochemistry and a fully delocalized CuI-CuII mixed valent state. The latter feature may be related to the short metal-metal separation of approximately 2.5 Angstrom units, which has been interpreted by some as indicating a direct metal-metal bonding interaction. Significantly, this claim is the first for a biologically occurring dimetallic entity involving a metal-metal bond, and as such a detailed theoretical understanding of the nature of the CuA site is warranted. Pursuant to this end, the preparation of dicopper complexes with the novel thiolate-containing cryptand ligands described herein will help to ascertain the features requisite for a functional CuA site. The cryptate ligand design is such as to permit variation in the number and type of donor atoms to the metals as well as adjustment of the separation between the distinct metal ion binding pockets. Variations in each of these aspects will permit separate assessment of their importance to the CuA site. Coupled with a theoretical bonding analysis of these model compounds, these synthetic studies will yield a more accurate description of the nature of the CuA center. More generally, the successful use of cryptands to bind two metal ions will make these ligands attractive for modeling other enzyme dimetallic sites. These cryptands may permit preparation of dinuclear structures which are otherwise difficult to prepare with simple ligands. Ultimately, the synthesis of new dimetallic structure types with biologically relevant types of donor ligands may provide specific spectroscopic criteria whereby the recognition of such structure in new metalloenzymes will be facilitated. Identification of the metal centers in new metalloenzymes is greatly aided by comparison to the properties of simple inorganic analogues.